set_chronos_password.sh: Add missing sudo while grepping the contents of shadow on root fs.

scripts/image_signing/set_chronos_password.sh

Index: scripts/image_signing/set_chronos_password.sh
diff --git a/scripts/image_signing/set_chronos_password.sh b/scripts/image_signing/set_chronos_password.sh
index f1b1ac380d6e63671916ee8c7ab89afd7f061fef..ff71e45867bf29f39bac66b1d2fcaf25b9b9f6af 100755
--- a/scripts/image_signing/set_chronos_password.sh
+++ b/scripts/image_signing/set_chronos_password.sh
@@ -19,7 +19,7 @@ change_chronos_password() {
   local temp_shadow="$rootfs/etc/tempshadow"
   echo "chronos:$crypted_password:14500:0:99999::::" \
     | sudo tee "$temp_shadow" > /dev/null
-  grep -Ev ^chronos: "$rootfs/etc/shadow" \
+  sudo grep -Ev ^chronos: "$rootfs/etc/shadow" \
     | sudo tee -a "$temp_shadow" > /dev/null
   sudo mv -f "$temp_shadow" "$rootfs/etc/shadow"
 }